Since propylene-based polymers have excellent mechanical properties such as rigidity and heat resistance and a good moldability, can be produced at relatively low costs, and exhibit a high adaptability to environmental problems, the polymers have been industrially applied to wide uses.
Therefore, the production process of polypropylene has been technically investigated continuously from the viewpoints of simplifying its steps, reducing production costs, improving productivity, and further improving catalyst performance.
In the production process of polypropylene, at the beginning of industrial production of polypropylene, catalyst performance was low and thus a step for removing a catalyst residue, atactic polymer, and the like was necessary, so that processes such as a slurry process using a solvent were mainly employed.
Thereafter, with remarkable advance in catalyst performance, the necessity of removing the catalyst residue, atactic polymer, and the like has been reduced. Currently, a vapor-phase process is a mainstream.
In the vapor-phase process, recently, highly activated type catalysts including supported catalysts as representatives have been commonly used. However, application of novel catalysts, such as highly activated catalysts and metallocene catalysts developed in recent progress in the catalyst technology, to the process and inhibition of generation of an aggregated polymer and reduction of formation of fine powder become problems to be solved also from the viewpoint of stable operation of the process.
For preventing the generation of the aggregated amorphous polymers in the vapor-phase polymerization, removal of heat of polymerization is relatively difficult in a catalyst feed part in a fluidized bed reactor, so that temperature in the fluidized bed is prone to be unstable by local accumulation of the heat of polymerization. Therefore, there has been proposed a production method of polypropylene wherein temperature and pressure in a vapor-phase polymerization reactor are controlled by a liquid flow rate of a liquefied circulating gas returning to the reactor, a flow rate of a discharging gas to the outside of the system, and a flow rate of a feeding monomer gas utilizing heat of vaporization using a fluidized bed reactor (see e.g., Patent Document 4). However, there is unavoidably a room for improvement in view of the removal of the heat of polymerization in the fluidized bed reactor at the time of increase in production and rapid grade change.
Moreover, for the purpose of inhibiting the formation of the aggregated polymer and retaining a fluidized state, there has been proposed a vapor-phase polymerization method characterized in that an inner wall temperature of a reactor is cooled to the dew point of a fluidizing gas or lower at polymerization in a fluidized bed reactor (see, e.g., Patent Document 5). However, the present inventors have recognized that difficulty is involved in operation control because of local phase change.
On the other hand, as a vapor-phase process, there is known a polymerization process using a horizontal polymerization reactor equipped with stirring vanes rotating around a horizontal axis as a vapor-phase reactor of an olefin in which heat of polymerization is removed utilizing heat of vaporization of liquefied propylene. The method for removing the heat of polymerization utilizing heat of vaporization of liquefied propylene has an advantage in view that a large heat-removing ability can be realized by a small facility.
Moreover, polymer particles polymerized in the horizontal polymerization reactor are formed in a reactor, transferred and advanced along the reactor by stirring with the progress of polymerization, so that there is exhibited a piston flow type that is a flow pattern in the case where several pieces of continuous stirred tank reactor are serially aligned. The horizontal polymerization reactor is economically advantageous in view that a solid mixing degree equal to the degree in the case of two or three or more reactors can be easily achieved with regard to a ratio of length to diameter.
Furthermore, since the polymerization reactor is a horizontal type, the reactor is advantageous in view of efficient removal of the heat of polymerization at heat removal as compared with a vertical reactor. A technique of removing the heat of polymerization utilizing the heat of vaporization of liquefied propylene and using a horizontal polymerization reactor equipped with a stirring vanes rotating around a horizontal axis has excellent characteristics as mentioned above (see, e.g., Patent Document 1).
Upon precise investigation on the process using the horizontal polymerization reactor, the present inventors have grasped the following problems caused by the generation of the aggregated and fine powder polymer.
In general, in the horizontal polymerization reactor, a catalyst is fed from the upstream end of the reactor to the inside of the reactor and propylene powder is formed in the reactor and is extracted from a downstream thereof. In the process where the heat of vaporization of liquefied propylene is utilized for removing the heat of polymerization, since a large amount of a vaporized gas is generated, it is desirable for the polymer particles to make a particulate shape as rapid as possible at an upstream part in order to avoid entrainment of fine powder with the vaporized gas to attach the powder to pipes and filters in a gas discharge pipe system or occlude them. On the other hand, acceleration of the reaction may cause generation of the aggregated amorphous polymers owing to local heat generation induced by the rapid reaction and may cause discontinuation of the production process.
In the case where the heat of vaporization of liquefied propylene is utilized, it is a common method that a gas is extracted from the reactor, the gas is cooled by a heat exchanger to liquefy it, and the liquefied gas is returned to the reactor. Since temperature at which gas is liquefied (dew point) depends on pressure and gas composition, when a gas component having a low dew point, such as hydrogen or ethylene, is mixed into propylene, the dew point is lowered as a mixing amount increases. Cooling ability of a heat exchanger is determined by a facility and, in the case where the same facility is used, an ability to liquefy a gas decreases, i.e., heat-removing ability decreases as the dew point of the gas component lowers.
Thus, in the production process where hydrogen or ethylene is present in the reactor in a large amount, such as a propylene-based polymer having a high MFR or a random polymer using a novel catalyst such as a highly activated catalyst or a metallocene catalyst, catalyst activity is enhanced but the generation of the aggregated polymer owing to the decrease in heat-removing ability is unavoidable. Moreover, since a high polymerization activity is exhibited, problems such as deterioration of the particle shape, formation of fine powder, and degradation of powder properties arise, so that there are problems in the application to the polymerization process.
As above, in the case where a novel catalyst such as a highly activated catalyst or a metallocene catalyst is applied to a vapor-phase polymerization for producing polypropylene, there are problems to be solved in view of inhibition of the aggregated amorphous polymers or fine powder and productivity.
In order to cope with such problems, there has been proposed a method for inhibiting a polymer aggregate by separating a feed port for a titanium-supported catalyst component from a feed port for a co-catalyst component in a horizontal polymerization reactor (see, e.g., Patent Document 2). Moreover, there has been also proposed a method for inhibiting aggregated amorphous polymers by a pre-polymerization treatment with an α-olefin and donor addition (see, e.g., Patent Document 3).
Furthermore, there has been proposed a method for producing an α-olefin polymer wherein temperature of the reactor becomes homogeneous and the generation of the aggregated amorphous polymers is prevented by performing operation at a pressure and temperature in a reaction zone within a certain pressure range under dew point curve in a pressure-temperature diagram, in a substantially vertical stirring bed reactor (see, e.g., Patent Document 6).
In the problem of the generation of fine powder, in the process utilizing the heat of vaporization of liquefied propylene, there has been mentioned a problem that the gas flow rate in the gas discharge pipe system is large due to the generation of a large amount of a vaporized gas, fine powder and fine particles of the polymer particles are entrained with the vaporized gas (entrainment phenomenon), to attach to or occlude the pipes and filters of the gas discharge pipe system. When the attachment and occlusion become excessive, it becomes unavoidable to stop the production process and perform cleaning. In order to avoid such a problem, there has been proposed a method of diminishing the amount of the fine powder by a method of providing a separating chamber through which an unreacted gas and cooled steam discharged from an upper part of a horizontal polymerization reactor are allowed to pass and spraying a liquid coolant into the separating chamber (see, e.g., Patent Document 7).